Light-sensitive silver halide photographic materials are comprised of hydrophilic colloid component layers containing various photographic additives. Said photographic additives are generally incorporated in the hydrophilic colloid compositions for forming said component layers by dissolving them in water or in water-miscible organic solvents and adding the resulting solution to said compositions.
Generally, however, many photographic additives are difficult to dissolve in water and, even when soluble in water-miscible organic solvents, they are incompatible with hydrophilic colloid compositions when incorporated therein through organic solvents. This is the case of many photographic additives which are rendered non- diffusible within the photographic layers by including a long hydrophobic carbon atom chain (the so called "ballasting chain") in their structural formula. Hydrophobic (ballasted) photographic additives include, for example, dye-forming couplers, DIR compounds, UV absorbers, anti-oxidants, image stabilizers, etc.
Typically, the process of incorporating such hydrophobic photographic additives into hydrophilic colloid components layers of photographic materials, such as silver halide emulsion layers, protective layers, intermediate layers and the like, consists of incorporating the photographic additives into hydrophilic colloid coating compositions for said layers. The -photographic additives are incorporated in the form of a dispersion of fine droplets consisting of a water-immiscible high boiling organic solvent in which said hydrophobic additives have been dissolved.
According to said dispersion technique, as described in U.S. Pat. No. 2,322,027, the hydrophobic photographic additives are generally dissolved in water-immiscible high boiling organic solvents (also called in the art permanent solvents, crystalloidal solvents, oil-type solvents, oil-formers and the like) and the resulting organic solution is added to an aqueous composition containing a hydrophilic colloid (gelatin) and a dispersing agent (surfactant). The mixture is then passed through a homogenizing apparatus (colloidal mill) to form a dispersion of fine droplets of said organic solvent containing the hydrophobic photographic additives. In some cases it may be advantageous to facilitate the dissolution of the additives by use of an auxiliary water-immiscible low boiling organic solvent, which is removed afterwards by evaporation, as described e.g. in U.S. Pat. Nos. 2,801,170, 2,801,171 and 2,949,360. The obtained dispersion is then mixed with the hydrophilic colloid composition (gelatin silver halide emulsion or other gelatin-containing composition) which is used to form {by coating) the photographic layer.
Organic solvents for dispersing -photographic additives are well known in the art, as disclosed for example in U.S. Pat. Nos. 2,322,027, 2,801,171, 2,835,579, 2,533,514, 3,554,755, 3,748,141, 3,799,765, 4,353,979, 4,430,421 and 4,430,422.
In particular, esters of aliphatic carboxylic acids have been described, for dispersing photographic additives, in U.S. Pat. No. 2,322,027 (such as tetrahydrofurfuryl succinate, ethyl benzyl malonate, .alpha.-naphthyl acetate), in U.S. Pat. No. 3,748,141 (such as quinitol di-2-ethylhexanoate and 1,4-cyclohexyl dimethylene-bis-2-ethylhexanoate) and in U.S. Pat. No. 3,779,765 (such as those corresponding to the formula ##STR2## wherein R.sup.VI is defined as an alkoxycarbonyl group having up to 15 carbon atoms, particularly two to 13 carbon atoms such as methoxycarbonyl or dodecyloxycarbonyl). Tetrahydrofurfuryl adipate has been reported in U.S. Pat. Nos. 2,801,171 and 2,949,360 as water-soluble organic solvent to be used as auxiliary solvent (in addition to the high boiling organic crystalloidal solvents) and removed from the emulsion by washing with water.
Organic solvents for dispersing hydrophobic photographic additives are required to meet several needs. They must (a) possess an excellent dissolving power towards said additives, (b) not cause crystallization of additives, (c) keep the fine droplets stably dispersed, (d) have a refractive index which is as close as possible to that of the hydrophilic colloid in which they are dispersed, and (e) not deteriorate the physical properties of the layers in which they are incorporated. Moreover, said organic solvents should not negatively affect the photographic properties of the photographic materials in which they are used to disperse photographic additives. For example, they must not give rise to fogging of the light-sensitive silver halide emulsions, and not negatively affect the stability during storage of dye-forming couplers (dispersed with said organic solvents) and of the dyes formed from said couplers during processing (stability to heat, humidity and light).
Accordingly, there is a continuous need for providing improved water-immiscible high boiling organic solvents for use in photography to disperse hydrophobic additives and for providing improved techniques for incorporating said additives into photographic layers.